HAL Id: tel-01127303
https://pastel.archives-ouvertes.fr/tel-01127303
Submitted on 7 Mar 2015
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
sand and clay using an environmental chamber
Weikang Song
To cite this version:
Weikang Song. Experimental investigation of water evaporation from sand and clay using an envi-ronmental chamber. Materials. Université Paris-Est, 2014. English. �NNT : 2014PEST1047�. �tel-01127303�
Pour obtenir le grade de
Docteur de l’Université Paris-Est
Discipline : Géotechnique
Présentée par
Weikang SONG
Experimental investigation of water evaporation from sand
and clay using an environmental chamber
Soutenue le lundi 10 Mars 2014 devant le jury composé de:
Prof. Pierre-Yves HICHER Ecole Centrale de Nantes Rapporteur Prof. De’an SUN Shanghai University Rapporteur Prof. Liangtong ZHAN Zhejiang University Examinateur Prof. Weimin YE Tongji University Examinateur Prof. Yu-Jun CUI Ecole des Ponts ParisTech Directeur de thèse Prof. Wenqi DING Tongji University Co-Directeur de thèse
Titre: Etude d'évaporation d'eau d'un sable et d'une argile à l'aide d'une chambre
environnementale
Il est bien connu que l'évaporation d'eau joue un rôle essentiel dans l'interaction entre le sol et l'atmosphère. Pendant le processus d'évaporation, le comportement thermo-hydro-mécanique des sols change, engendrant ainsi des problèmes préoccupants. Ceci peut concerner différents domaines comme l'agronomie, l'hydrologie, la science des sols, la géotechnique, etc. Par conséquent, il est essentiel d'étudier les mécanismes d'évaporation de façon approfondie.
Cette étude porte sur les mécanismes d'évaporation dans des conditions atmosphériques contrôlées. Le sable de Fontainebleau et l'argile d’Héricourt utilisée pour la construction du remblai expérimental dans le cadre du projet ANR TerDOUEST (Terrassements Durables - Ouvrages en Sols Traités, 2008-2012) ont été étudiés à cet effet. Une chambre environnementale (900 mm de haut, 800 mm de large et 1000 mm de long) équipée de différents capteurs a d'abord été développée, permettant un suivi complet des paramètres concernant l'atmosphère et le sol au cours d'évaporation.
Quatre essais expérimentaux ont été réalisés sur le sable de Fontainebleau compacté à
une densité sèche de 1,70 Mg/m3, avec une nappe phréatique constante au fond de
l'échantillon, et sous différentes conditions atmosphériques (différentes valeurs de l'humidité relative de l'air, de la température et du débit d'air). La pertinence du système a été mise en évidence par la bonne qualité des résultats. La température de l'air à l'intérieur de la chambre a été trouvée affectée par la température du tube de chauffage, le débit d'air et l'évaporation d’eau; la température du sol est fortement affectée par les conditions atmosphériques et l'état d'avancement de l'évaporation; l'humidité relative dans la chambre diminue au cours du temps et son évolution peut être considérée comme un indicateur du processus d'évaporation; la teneur en eau volumique dans la zone proche de la surface est fortement influencée par le processus d'évaporation et présente une relation linéaire avec la profondeur; la succion du sol
vitesse d'évaporation constante.
Après les essais sur le sable de Fontainebleau, l'échantillon de l'argile d'Héricourt
compactée à une densité sèche de 1,40 Mg/m3 a été soumis à une infiltration d’eau
afin d'étudier ses propriétés hydrauliques. Pour obtenir un meilleur aperçu du mécanisme d'évaporation pour l'argile, deux essais d'évaporation sur l'argile d'Héricourt compactée avec une nappe phréatique constante au fond de l’échantillon ont été effectuées sous des conditions atmosphériques contrôlées. Les résultats permettent de comprendre les mécanismes d'évaporation en cas de fissuration due à la dessiccation. En outre, afin d'étudier les mécanismes d'évaporation potentiels, des essais avec une couche d'eau libre ont été également réalisés en faisant varier la vitesse du vent et la température de l'air. L'initiation et la propagation de fissures de dessiccation pendant le processus d'évaporation et son effet sur l'évaporation ont également été étudiés par la technique de traitement d'image.
En termes de modélisation, le taux d'évaporation potentiel a été modélisé à travers l'évaluation des modèles existants et des modèles combinés. Il apparait que le modèle développé par Ta (2009) est le plus approprié. Le taux d'évaporation réelle depuis le sable a été ensuite analysé. Il semble important de considérer l'avancement du front sec pendant le processus d'évaporation pour les sols sableux. Pour l'argile d'Héricourt, une bonne prévision a été également obtenue en utilisant un modèle qui tient compte de l'effet des fissures de dessiccation.
Mots clés:
mécanism d'évaporation; sable; argile; chambre environnementale;conditions atmosphérique; fissuration de dessicccation; évaporation potentielle; évaporation réelle; modèle d'évaporation
Title: Experimental investigation of water evaporation from sand and clay using an
environmental chamber
As a well-known phenomenon, soil water evaporation plays an important role in the interaction between soil and atmosphere. Water evaporates during this process resulting in changes of soil thermo-hydro-mechanical behavior and in turn causing problems in different domains such as agronomy, hydrology, soil science, geotechnical engineering, etc. Therefore, it is essential to investigate the soil water evaporation mechanisms in depth.
This study deals with the soil water evaporation mechanisms under controlled atmospheric conditions. The Fontainebleau sand and the Héricourt clay used for the construction of the experimental embankment with the ANR project TerDOUEST (Terrassements Durables - Ouvrages en Sols Traités, 2008 - 2012) were used in this investigation. A large-scale environmental chamber system (900 mm high, 800 mm large and 1000 mm long) equipped with various sensors was firstly developed, allowing a full monitoring of both atmospheric and soil parameters during the evaporation process.
Four experimental tests were carried out on the Fontainebleau sand compacted at
1.70 Mg/m3 dry density with a steady water table at soil bottom under different
atmospheric conditions (different values of air relative humidity, temperature and air flow rate). The performance of the environmental chamber system in investigating soil water evaporation was evidenced by the quality and the relevance of results. The air temperature inside the chamber was found to be affected by the heating tube temperature, the air flow rate and the soil water evaporation process; the soil temperature was strongly affected by the air conditions and the evaporation progress; the relative humidity in the chamber was decreasing during the evaporation progress and its evolution could be considered as an indicator of the evaporation progress; the volumetric water content in the near-surface zone was strongly affected by the evaporation process and exhibited a linear relationship with depth; the soil suction
After the tests on the Fontainebleau sand, the Héricourt clay sample compacted at
1.40 Mg/m3 dry density was subjected to an infiltration experiment for investigating
its hydraulic properties. To get a better insight into the water evaporation mechanism for clay, two compacted Héricourt clay evaporation tests with a steady water table at bottom were carried out under controlled atmospheric conditions. The results allow understanding the evaporation mechanisms in case of desiccation cracks. Furthermore, in order to investigate the potential evaporation mechanisms, tests with a free water layer was also conducted with varying wind speed and air temperature. The initiation and propagation of desiccation cracking during the evaporation process and its effect on water evaporation were also investigated by the digital image processing technique.
In terms of modeling, the potential evaporation rate was first modeled through evaluation of the existing models and the combined models. It reveals that the model developed by Ta (2009) is the most appropriate one. The actual evaporation rate for sand was then analyzed. It appears important to consider the progress of the dry front during the evaporation process for sandy soils. For the Héricourt clay, good simulation was also obtained using a model that accounts for the effect of desiccations cracks.
Keywords:
evaporation mechanism; sand; clay; environmental chamber;atmospheric conditions; desiccation cracking; potential evaporation; actual evaporation; evaporation model
题目：砂土与粘土水分蒸发机理的环境箱试验研究 众所周知，土体水分蒸发是土与大气交互作用过程中的一个重要环节。在这 个过程中，土中水分的蒸发引起了土体热-水力-力学性质的变化，从而在诸如农 学、水文学、土壤学和岩土工程等领域引起了各种各样的问题。因此，对土体水 分的蒸发机理进行研究至关重要。 本文主要对控制大气条件下土体水分的蒸发机理进行研究。所采用试验材料
为Fontainebleau 砂土和用于法国基金委（ANR）项目 TerDOUEST（Terrassements
Durables - Ouvrages en Sols Traités, 2008-2012）中试验路堤建设的 Héricourt 粘土。 为了能够对蒸发过程中的大气和土体参数进行全方位的监测，本文开发了一个配 备多种传感器的大体积环境箱蒸发测量系统（尺寸：1000 mm 长，800 mm 宽， 900 mm 高）。随后进行了 4 组不同大气条件下（不同相对湿度、不同温度和空气 流量）底部保持稳定水位的大体积压实Fontainebleau 砂土（干密度 1.7 g/cm3） 蒸发试验。高质量的试验结果验证了环境箱蒸发测量系统的工作性能。研究结果 还表明：环境箱内空气温度的变化受到加热管温度、空气流量和土体水分蒸发过 程的影响；土体温度的变化也深受大气条件和蒸发过程的影响；在蒸发过程中， 环境箱内空气的相对湿度随着蒸发的进行逐渐降低，它的变化可以看作蒸发过程 的一个指示器；表层区域内土体体积含水量的变化受蒸发过程的影响较大；其分 布与土体深度呈线性关系；土体吸力沿着深度方向逐渐降低，但随蒸发时间的增 加而增大；蒸发速率特别是在初始常速率阶段受到大气条件变化的影响较大。而 后，为了研究 Héricourt 粘土的水力性质，在该环境箱内进行了大体积压实 Héricourt 粘土（干密度为 1.4 g/cm3）的渗透试验。此外，为了更加深入地对粘 性土土体水分蒸发机理进行研究，在渗透试验结束后，又进行了2 组保持土体底 部稳定水位、控制大气条件的 Héricourt 粘土水分蒸发试验。该试验研究结果可 以加深对龟裂条件下土体水分蒸发机理的理解。为了研究潜在蒸发的机理，在该 环境箱内进行了自由水面在不同风速和空气温度条件下的蒸发试验。另一方面， 通过数字图像处理技术对水分蒸发过程中龟裂的开始与演化，以及龟裂对蒸发过 程的影响进行了研究。最后，在蒸发试验的基础上对蒸发速率的计算模型进行研 究。对潜在蒸发而言，本文通过对已有的计算模型及不同模型的组合的研究，验 证了 Ta（2009）所提出的模型的适用性。针对砂土水分蒸发的特点，本文提出 并验证了一个可以考虑蒸发过程中干燥面变化的蒸发速率计算模型。此外，通过 对粘土水分蒸发机理的研究，本文建立并验证了一个新的能够考虑龟裂影响的蒸
关键词：蒸发机理，砂土，粘土，环境箱，大气条件，龟裂，潜在蒸发，实际蒸 发，蒸发模型
My deepest gratitude goes first and foremost to Prof. Yu-Jun Cui, my supervisor at Ecole des Ponts ParisTech and Universite Paris-Est, for his constant encouragement and guidance during the whole period of this dissertation. Without his profound scientific vision, valuable advices, patient instruction, this thesis could not have reached its present form. The experiences that work with him are a precious treasure in my lifetime and also make me go farther on the scientific road.
I would like to express my deep and sincere gratitude to Prof. Wenqi Ding, my supervisor at Tongji University, for his consistent patience and encouragement throughout all my study period at Tongji University. His systematic guidance and constant encouragement are invaluable assets for completing this dissertation.
I would like to express my deep appreciation to Dr. Anh Minh Tang for his patient instructions and valuable suggestions. His deep understanding and great experience in laboratory testing gave me great helps in overcoming the problems I met in the laboratory. The extended discussions with him and the careful revision by him have tremendous contribution to my scientific publications.
I would like to express my heartfelt gratitude to professors both in France and China:
Prof. Pierre Delage, Prof. Martine Audiguier, Prof. Roger Cojean, Prof. Chaosheng
Tang, Prof. Weimin Ye, Prof. Yonggui , Prof. Bao Chen and Prof. Xiangdong Hu. The dissertation has benefited from their comments and suggestions. I also record my appreciation to Prof. Yuancheng Guo for his constant encouragement.
The experimental work would not have been possible without the constant assistance of the technical team of CERMES: Emmanuel De Laure, Xavier Boulay, Hocine Delmi, Baptiste Chabot, Clapies Thomas. Their serious working attitude and sophisticated technique are the assurance to meet the goal of my experimental work. I would like to specially thank Dr. Qiong Wang, Dr. Yawei Zhang and Dr. Thanh Danh Tran for their altruistic assistances and constant encouragements during my experiment and the writing of dissertation. Countless valuable experimental
give a great inspiration to complete my laboratory work. My gratitude goes also to Dr. Pengyun Hong, Dr. Jucai Dong, Dr. Kun Li, Dr. Linlin Wang for their encouragement and support.
I would like to especially thank the reviewers of my dissertation, Prof. Pierre-Yves HICHER at Ecole Centrale de Nantes, who also presided over the jury, and Prof. De’an SUN at Shanghai University, for agreeing to take the time to review my dissertation carefully and to give insightful comments on my thesis. The same gratitude goes to the examiners: Prof. Liangtong ZHAN at Zhejiang University and Prof. Weimin YE at Tongji University.
My sincerest thanks go also to everyone in CERMES for their encouragement, support and all the nice times I had with them. My thanks also go to all my friends who worked in other laboratories in France and helped me directly or indirectly in the successful completion of my dissertation.
I am also grateful to Ecole des Ponts Paris-Tech, the China Scholarship Council (CSC) and the TerDOUEST project for their supports.
Finally, my thanks would go to my beloved parents, brother and sister for their understanding and supporting during my life. Without their love and support, I would not be the same person that I am today. Last but certainly not least, I will owe my sincere gratitude to my girl friend, Miss Shan XU. Without her deep love and consistent encouragement, my work would not have been possible.
1. Song, W.K., Cui, Y.J., Tang, A.M., and Ding, W.Q., 2013. Development of a large-scale environmental chamber for investigating soil water evaporation. Geotechnical Testing Journal, 36(6): 847-857. doi:10.1520/GTJ20120142.
2. Song, W.K., Cui, Y.J., Tang, A.M., Ding, W.Q., and Tran, T.D., 2013. Experimental study on water evaporation from sand using environmental chamber. Canadian Geotechnical Journal, 51(2): 115-128. doi: 10.1139/cgj-2013-0155. 3. Tran, T.D., Song, W.K., Audiguier, M., Cojean, R., Tang, A.M., and Cui, Y.J.,
2013. Investigating the microstructure of clayey soil under wetting and/or drying. Submitted to Engineering Geology.
4. Song, W.K., Ding, W.Q., Cui, Y.J., 2014. Model test study of evaporation
mechanism of sand under constant atmospheric condition. Chinese Journal of Rock Mechanics and Engineering, 33(2):405-412. ( in Chinese)
5. Song, W.K., Ding, W.Q., Cui, Y.J., 2014. Experimental investigation of water evaporation from Fontainebleau sand using an environmental chamber. Water Resource Research, 25(1): 69-76. (in Chinese)
INTRODUCTION...1
1. Research background and significance...1
2. Objectives and organization of the thesis ...2
Chapter 1 Water evaporation from soil: models, experiments and applications ...5
1.1 Phenomenon of evapotranspiration...5
1.1.1 Evaporation ...5
1.1.2 Transpiration ...6
1.1.3 Evapotranspiration ...6
1.1.4 Potential and actual evaporation ...6
1.2 Soils water evaporation experiments ...7
1.2.1 Introduction...7
1.2.2 Advance in evaporation experimentation...7
1.2.3 Discussions ...20
1.3 The process of soil water evaporation and its influencing factors...21
1.3.1 The requirements for the initiation of evaporation ...21
1.3.2 The typical process of evaporation ...21
1.3.3 The factors influencing soil water evaporation...24
1.4 Modeling of soil water evaporation ...36
1.4.1 Introduction...36
1.4.2 Water balance model ...37
1.4.3 Energy balance model...37
1.4.4 The mass transfer model ...41
1.4.5 The resistance model...42
1.4.6 Coupled models ...47
1.4.7 Recent models...50
1.4.8 Conclusions...56
1.5 Recent applications with consideration of soil water evaporation...57
1.5.1 Introduction...57
1.5.2 Geotechnical applications ...58
1.6 Conclusions...62
Chapter 2 Materials studied and environmental chamber developed ...65
2.1 Introduction...65
2.2 Materials studied...66
2.2.2 Héricourt clay...67
2.3 The environmental chamber developed ...72
2.3.1 Description of the environmental chamber...72
2.3.2 Description of the sensors used...82
2.3.3 Experimental procedures ...97
2.4 Discussion ...98
2.5 Conclusion ...99
Chapter 3 Evaporation test on Fontainebleau sand ...101
3.1 Introduction...101 3.2 Experimental methods ...104 3.2.1 Test procedure ...104 3.2.2 Test program ...105 3.3 Results...106 3.3.1 Test 1 ...106 3.3.2 Test 2 ... 114 3.3.3 Test 3 ...122 3.3.4 Test 4 ...130
3.4 Comparative analysis of the results from the four tests...139
3.4.1 Air temperature ...139
3.4.2 Soil temperature ...140
3.4.3 Air-soil profile...140
3.4.4 Relative humidity...141
3.4.5 Volumetric water content ...142
3.4.6 Suction ...142
3.4.7 Evaporation rate ...144
3.5 Discussions ...144
3.6 Conclusions...151
Chapter 4 Evaporation test on Héricourt clay ...155
4.1 Introduction...155
4.2 Experimental methods ...158
4.2.1 Soil preparation...158
4.2.2 Soil compaction and sensors installation ...159
4.2.3 Infiltration test...162
4.2.4 Evaporation test ...163
4.2.5 Investigation of soil surface cracking ...166
4.3 Results...170
4.3.1 Soil infiltration test ...170
4.4 Discussions ...224
4.4.1 Evolution of soil parameters during infiltration...224
4.4.2 Evolutions of soil and air parameters during evaporation ...226
4.4.3 The initiation of soil cracks...230
4.4.4 The critical water contents ...231
4.4.5 The three stages of crack evolution...232
4.4.6 Changes of quantitative analysis parameters ...233
4.4.7 Evolution of crack pattern...236
4.5 Conclusions...238
4.5.1 Infiltration test...238
4.5.2 Soil water evaporation test...238
4.5.3 Soil desiccation cracking ...241
Chapter 5 Modelling of potential water evaporation ...245
5.1 Introduction...245
5.2 Assessment of existing models ...246
5.3 Comparison between various models ...247
5.3.1 Model 1 ...247 5.3.2 Model 2 ...249 5.3.3 Model 3 ...250 5.3.4 Model 4 ...251 5.3.5 Model 5 ...252 5.3.6 Model 6 ...254 5.3.7 Model 7 ...255 5.3.8 Model 8 ...257 5.3.9 Extended models...259
5.4 Application to the soil water evaporation tests ...264
5.4.1 Héricourt clay evaporation test ...264
5.4.2 Fontainebleau sand evaporation test ...268
5.5 Conclusion ...275
Chapter 6 Modelling of actual water evaporation from sand and clay ...277
6.1 Introduction...277
6.2 Model for water evaporation from sand...278
6.2.1 Verification of the selected suction related models...278
6.2.2 Modification of the existing suction related model ...280
6.2.3 Verification of the new actual evaporation model ...287
6.3 Model for water evaporation from clay ...291
6.3.1 Proposed model...291
6.3.2 Parameters determination and model verification ...293
6.3.3 Discussion ...299
6.4 Conclusions...300
GENERAL CONCLUSION...303
INTRODUCTION
1. Research background and significance
Soil water evaporation is an important energy exchange process and water cycle component. It causes a lot of problems in various fields: soil degradation in arid area with high evaporation rate (Xue and Akae, 2012), soil salinization in arid and
semi-arid regions (Shimojima et al., 1996; Zarei et al., 2009; Xue and Akae, 2012),
damage of buildings and geotechnical constructions due to water loss (Cui and Zornberg, 2008; Corti et al., 2009; Corti et al., 2011), affecting the potential performance and the safety of the high-level nuclear waste repository due to the desaturation process induced by the forced ventilation in galleries and drifts during the construction and operation phases (Bond et al., 2013; Millard et al., 2013), etc. This shows the importance of investigating the mechanisms of soil water evaporation. On the other hand, the study of this process has significant practical benefits in various fields: estimating the amount of water loss in the assessment of soil management technologies in agriculture (Qiu et al., 1998), predicting evaporation flux in design of soil cover of mine tailings (Wilson 1990; Wilson et al., 1994; Wilson et al., 1997; Yanful and Choo, 1997), investigating the long term performance of moisture retaining soil covers (Yang and Yanful, 2002; Yanful et al., 2003), designing evapotranspirative cover systems for waste containment and mining sites (Cui and Zornberg, 2008), classifying landfill sites according to the climatic water balance (Blight, 2009), etc. Moreover, the investigation of soil water evaporation is also an important issue in geotechnical engineering (Cui et al., 2010; Cui et al., 2013).
In this context, number of laboratory studies has been conducted to investigate the soil water evaporation process (Wilson et al., 1994; Wilson et al., 1997; Yanful and Choo, 1997; Yamanaka et al., 1997; Aluwihare and Watanabe, 2003; Smits et al., 2011). However, the water evaporation from soil depends not only on the atmospheric conditions but also on the soil properties. Most of the existing studies mainly focus on part of the related parameters. The comprehensive study on both soil and atmospheric parameters during evaporation has rarely been undertaken. As far as the model for predicting water evaporation is concerned, the existing models mainly consider the
effect of atmospheric parameters and soil water content (Blight, 1997; Burt et al.,
2005;Cui and Zornberg, 2008; Penman, 1948; Monteith, 1981; Singh and Xu, 1997).
These types of models are not easy to be used in the prediction of soil deformation resulting from water evaporation, because of the difficulty in defining the boundary conditions. On the other hand, the suction related model (Wilson et al., 1997; Aydin et al., 2005; Ta, 2009) seems to be a promising model for this purpose. Nevertheless, the influence of soil cracks remains a challenge in case of clayey soils submitted to desiccation.
In this context, an in-depth study on the soil water evaporation mechanism is conducted in this thesis. A large scale environmental chamber was developed for this purpose, allowing evaporation testing on soil samples under controlled atmospheric conditions and with monitoring of soil parameters such as suction, volumetric water content and temperature. In case of soil cracking, a camera is used for monitoring of cracks. Two soils are considered, the Fontainebleau sand and the expansive Héricourt clay. The results obtained allow the assessment of existing model for the potential evaporation description, and the development of actual evaporation models for sand and clays. Emphasis is put on the effect of the dry front in the case of sand and the effect of desiccation cracks in the case of clay.
2. Objectives and organization of the thesis
The main objective of the present investigation is to advance the knowledge on the evaporation mechanism of different soils under different atmospheric conditions. The more specific objectives are:
1. To develop a large-scale environmental chamber for investigating the soil water evaporation in-depth.
2. To further investigate the potential evaporation rate.
3. To investigate the Fontainebleau sand evaporation process under four different atmospheric conditions.
4. To investigate the Héricourt clay evaporation process under controlled atmospheric conditions.
5. To investigate the initiation and propagation of desiccation cracking during soil water evaporation process.
6. To propose relevant soil water evaporation models for both sand and clay. The thesis includes six chapters.
Chapter 1 provides an overview of the current knowledge on the soil water evaporation. The first part of this chapter recalls the basic concepts of evaporation. The second part of this chapter presents the common experimental techniques used in investigating soil water evaporation, with the advance in various experimental apparatus and the comparison between them. The third part of this chapter presents the process of soil water evaporation and the factors influencing soil water evaporation. The fourth part of this chapter introduces the current stage in soil water evaporation modeling. Different models are presented, including the water balance model, the energy balance model, the mass transfer model, the resistance model, the coupled model. The fifth part of this chapter introduces the soil evaporation related applications in geotechnical engineering, including the soil covers design, the damage assessment of buildings due to drought, the analysis of the effect of climate changes on the behavior of embankment and in the climatic classification of landfills.
Chapter 2 is devoted to the presentation of the large-scale environmental chamber used for investigating soil water evaporation. In this chapter, the composition of this environmental chamber is firstly introduced, together with the application and calibration procedure of the sensors installed in this chamber. Thereafter, the experimental procedure is defined and presented.
Chapter 3 presents the large-scale evaporation experiment conducted on the Fontainebleau sand. In this chapter, four sand evaporation tests under different atmospheric conditions and various drying durations are presented. The evolutions of the atmospheric parameters (air flow rate, relative humidity and temperature) and the response of soil (volumetric water content, temperature, soil suction) are investigated simultaneously. In addition, the performance of this chamber is assessed based on the experimental results.
Chapter 4 focus on the water evaporation from Héricourt clay under controlled atmospheric conditions. In this chapter, the response of Héricourt clay during
infiltration test is firstly illustrated. After that, the evolutions of both air and soil parameters in the first evaporation test is presented. Finally, for investigating the effect of cracks on soil water evaporation, a second evaporation test is conducted under the same conditions, and the results are also presented in this chapter.
Chapter 5 deals with the modeling of the potential evaporation rate. For this purpose, the existing models as well as the combinations of some existing models are evaluated based on the test results obtained in the case of free water evaporation and those during the constant rate stage of the evaporation tests. The appropriate model is then chosen for the further development.
Chapter 6 is devoted to the development of the water evaporation for sand and clay. A suction related model is taken as the basis. The simulation results show that this kind of models is relevant in describing soil water evaporation process provided that the progress of the dry front in the case of sand and the effect of cracks in the case of clay are taken into account.
Chapter 1 Water evaporation from soil: models,
experiments and applications
1.1 Phenomenon of evapotranspiration
1.1.1 Evaporation
Evaporation is a natural phenomenon and an important component of water hydrologic cycle. Liquid water is changed to vapor during the evaporation process. Freeze (1969) gave a definition of evaporation as: the removal of water from the soil
at the ground surface, together with the associated upward flow. However, this
definition does not refer to the mechanisms or origins of vapor flow (Wilson, 1990). Wilson (1990) considered that the term evaporation usually refers to free water and bare soil surface. Accordingly, under certain interior (inside soil mass) and external (atmosphere) conditions, the process involving liquid water changing to vapor and then entering the atmosphere is termed as soil water evaporation. The soil water evaporation is affected by both atmospheric conditions and soil properties; the different influential factors will be discussed in the next section. The evaporation-related hydrologic cycle is shown in Fig. 1.1.
1.1.2 Transpiration
The definition of transpiration given by Wilson (1990) is “The process by which water
vapour is transferred to the atmosphere from water within plants”. Burt et al. (2005)
gave a description of transpiration: “a specific form of evaporation in which water
from plant tissue is vaporized and removed to the atmosphere primarily through the plant stomata”. Cui and Zornberg (2008) termed transpiration as the evaporation from
the vascular system of plants. Considering different definitions above, a simple term can be adopted as follows: transpiration is water evaporation from plants (Hillel, 2004). The transpiration in the hydrologic cycle is presented in Fig. 1.1.
1.1.3 Evapotranspiration
Evapotranspiration is the combination of evaporation and transpiration. Wilson (1990) considered that the term evapotranspiration is the combination of water evaporation from host soil and the transpiration from the individual plants within the canopy. Similarly, Burt et al. (2005) pointed out that “the combined water that is transferred
to the atmosphere through evaporation and transpiration processes is known as evapotranspiration”.
1.1.4 Potential and actual evaporation
In general, the potential evaporation is considered as the maximum evaporation rate when water evaporates from pure water surface under certain climatic conditions (Wilson et al., 1994). As mentioned by World Meteorological Organization (WMO) (2006), the International Glossary of Hydrology (WMO/UNESCO, 1992) and the International Meteorological Vocabulary (1992) gave the definition as “Quantity of
water vapour which could be emitted by a surface of pure water, per unit surface area and unit time, under existing atmospheric conditions”.
According to the literatures, the rate of evaporation from pure water under the same conditions as from soil is considered as the potential evaporation rate (Wilson, 1990; Wilson et al., 1994; Wilson et al., 1997; Yanful and Choo, 1997; Lee et al., 2003; Shokri et al., 2008). This concept is adopted in this study. Accordingly, the direct measurement of evaporation rate from soil is termed as actual evaporation rate.
1.2 Soils water evaporation experiments
1.2.1 Introduction
Many devices have been developed to study soil water evaporation: evaporation pan, soil pan, soil column testing system, lysimeter, wind tunnel, environmental chamber etc. In this section, all these devices are summarized, and comparisons are made. Finally, a promising device is selected for the present study.
1.2.2 Advance in evaporation experimentation
The evaporation pan (Fig. 1.2) is usually used in field conditions for the measurement of free water evaporation that is considered as potential soil water evaporation (Blight, 1997; Singh and Xu, 1997; Fu et al., 2004; Fu et al., 2009; Li et al., 2011). Furthermore, the small evaporation pan was also used for the measurement of potential evaporation in the laboratory (Wilson et al., 1994; Wilson et al., 1997). In addition to the evaporation pan, the evaporation tank is a similar but bigger instrument
for the investigation of free water surface evaporation (e.g., Russian 20 m2
evaporation tank, see Fig. 1.3), but it is more expensive to build and maintain and can only be used at limited number of experimental stations (Fu et al., 2009).
For the soil water evaporation investigation, several simple devices have been developed. A circular pan with 300 mm in diameter but different heights and filled with compacted soil was used outdoor by Kondo et al. (1990, 1992) (see Figs. 1.4 and 1.5) for monitoring soil water evaporation. The evaporation rate was obtained directly by weighing the pan over time. When the soil height is small (20 mm), only global water content and soil surface temperature can be monitored during the test (Kondo et al., 1990). However, the global water content is different from the soil surface one. To minimize this difference, Wilson et al. (1997) studied soil water evaporation using three thin soil samples, i.e., 0.2 mm to 0.7 mm thick in a pan of 258 mm in diameter and 74 mm in height (see Fig. 1.6). In order to have a further insight into the soil response through the water content profile, thick samples should be used. Kondo et al. (1992) used soil samples with 100 and 130 mm in height but only the final water content profile was obtained by oven-drying, the temperature having been monitored automatically at different depths. Wilson et al. (1994) performed a drying test using a
soil column (i.e., 169 mm outside diameter and 300 mm high), allowing also automatically monitoring soil temperature, the water content having been monitored over time by direct measurement via sampling ports (see Fig. 1.7). On the other hand, other column drying test systems have been developed during these years and the evaporation rate was determined by measuring the mass change of soil column. The soil column evaporation test system (column dimension: 115 mm in diameter and 255 mm in height) developed by Yang and Yanful (2002) was used for investigating soil evaporation under different water table conditions, and this system allowed the measurement of volumetric water content and temperature simultaneously (see Fig. 1.8). The column drying test system (column dimension: 300 mm outside diameter and 800 mm high) proposed by Lee et al. (2003) was used to measure evaporation from deformable soils. The evolutions of suction, temperature and water content can be observed automatically during the test (see Fig. 1.9). More recently, a large soil column evaporation system (column dimension: 102 mm inside diameter and 1200 mm high) was developed by Smits et al. (2011) for investigating the sand water evaporation under controlled uniform and constant surface temperature conditions. It is noted that the soil water content, suction and temperature can be monitored continuously in this system (see Fig. 1.10).
Fig. 1.3. 20 m2 evaporation tank
(http://www.igsnrr.cas.cn/xwzx/tpxw/201007/t20100702_2891495.html)
Fig. 1.4. Soil pan filled with 20 mm height soil sample (Kondo et al., 1990)
Fig. 1.8. Column evaporation test system (Yang and Yanful, 2002)
Fig. 1.9. Column drying test system (Lee et al., 2003)
The lysimeter is another kind of popular equipment for measuring soil water evaporation in the field (Qiu et al., 1998; Benson et al., 2001; Liu et al., 2002; Benli et al., 2006) or in the laboratory (Bronswijk, 1991). Weighing and non-weighing are two widely used types of lysimeter. Weighing lysimeters (see Fig. 1.11) allow direct measurement of evaporation through changes in total mass of soil and the stored water can be measured (Fayer and Gee, 1997; Benson et al., 2001). In order to make the in-situ measurement simpler and more accurate, micro-lysimeter were developed (Boast and Robertson, 1982; Plauborg, 1995; Wang and Simmonds, 1997; Bonachela, 1999; Liu et al., 2002). Micro-lysimeters can also be combined with some water content sensors like TDR (time domain reflectometry) for the water evaporation monitoring (Wythers et al., 1999).
Fig. 1.11. Weighing lysimeter used in final cover studies (adapted from Fayer and Gee, 1997)
Atmospheric conditions (solar radiation, wind velocity, air temperature and relative humidity, etc.) are important factors governing soil water evaporation. A better control of atmospheric conditions is obviously essential in investigating soil water evaporation mechanisms. In this regard, the wind tunnel system is a good example. Typically, this system allows not only the control of wind velocity and solar radiation, but also the monitoring of air temperature and relative humidity (Yamanaka et al. 1997, Komatsu 2003, Yamanaka et al. 2004, Yuge et al. 2005, Wang 2006). This system can be used in combination with the experimental devices mentioned above like pan (Komatsu, 2003) (see Fig. 1.12), soil tank (Wang, 2006) (see Fig. 1.13), weighing lysimester (Yamanaka et al., 1997; Yamanaka et al., 2004) (see Figs. 1.14 and Fig. 1.15), micro-lysimeter (Yuge et al., 2005) (see Fig. 1.16) and soil column
(Shahraeeni et al., 2012) (see Fig. 1.17). Furthermore, if some sensors are used for soil temperature, suction and volumetric water content monitoring, this system allows a comprehensive monitoring of parameters for studying soil water evaporation (Yamanaka et al., 1997; Yamanaka et al., 2004).
Fig. 1.12.Wind tunnel experiment device (Komatsu, 2003)
Fig. 1.13. Wind tunnel experimental apparatus: (a) photograph of wind tunnel; (b) sketch of the wind tunnel and soil tank (Wang, 2006)
Fig. 1.14. The sketch of NIED wind tunnel system with weighing lysimeter (Yamanaka et al., 1997)
Fig. 1.15. The sketch wind tunnel system with weighing lysimeter (Yamanaka et al., 2004)
Fig. 1.17. Photograph of wind tunnel system with soil column (Shahraeeni et al., 2012)
Another commonly used system is the environmental chamber. A fast air circulation
box (dimensions: 800 mm×440 mm×400 mm, see Fig. 1.18) was developed by
Kohsiek (1981) with the simulation of wind. It is a useful chamber for the measurement of stomatal resistance of grass. After some minor adjustments (dimensions: 1000 mm × 400 mm × 800 mm) and equipment of a fast dry and wet
bulb thermocouple and a thermal infrared radiometer,this box was then used for soil
surface resistance investigation (see Fig. 1.19) (van de Griend and Owe, 1994). Watanabe and Tsutsui (1994) measured soil water evaporation using a ventilated chamber. The main principle of this chamber is based on the principle that changes in absolute humidity at inlet and outlet of the environmental chamber correspond to soil water evaporation. A transparent chamber was placed on the ground surface, and air was injected from one side and collected on the other side; meanwhile, the air relative humidity and temperature were monitored. This allows the water evaporation rate to be determined (Mohamed et al., 2000). This type of chamber can ensure a good control of atmospheric conditions, especially for the wind velocity distribution. Mohamed et al. (2000) developed a new chamber for predicting solute transfer in unsaturated sand due to evaporation (see Fig. 1.20). This chamber consists of a ventilated part and a soil part and the equipment developed by Watanabe and Tsutsui (1994) was used for evaporation measurement. Aluwihare and Watanabe (2003) developed an evaporation chamber system to study the surface resistance of bare soil
(see Fig. 1.21). On the whole, these chambers focus on the control of atmosphere conditions, such as wind speed, relative humidity, temperature etc., but rarely account for the soil parameters such as water content and suction. Yanful and Choo (1997) performed an evaporation experiment on a compacted cover soil using cylindrical columns placed in an environmental chamber (see Fig. 1.22). This chamber can control air temperature and relative humidity and measure soil temperature and water content at different depths during evaporation. However, the soil mass, temperature and water content measurements should be performed outside the chamber, the instantaneous and continuous measurements being not possible. Tang et al. (2009) developed a large-scale infiltration tank allowing instantaneous monitoring of soil water content, temperature and suction during evaporation (Ta, 2009; Ta et al., 2010; Cui et al., 2013)(see Fig. 1.23).
Fig. 1.18. Sketch of fast air circulation box (Kohsiek, 1981) (S is the partitions; P is the propeller; M is external electromotor; H is the holder for mounting the thermocouples; I and U are openings;
D is the opening for the feed-through of thermocouple wires)
Fig. 1.20. Sketch of ventilated chamber for evaporation measurement (Mohamed et al., 2000)
Fig. 1.22. Plan view of the environmental chamber system (Yanful and Choo, 1997)
1.2.3 Discussions
Evaporation pan is widely used in the prediction of water surface evaporation in field, the value it measures is considered to be the maximum evaporation rate. It is noted that the measured value from evaporation pan is affected by many factors such as the size, colour, depth, material, installation mode, structures and position (Fu et al., 2004; Fu et al., 2009). The soil column drying testing systems usually determine the evaporation rate through weighing the mass loss of the column. The soil responses to evaporation are monitored continually, such as volumetric content and temperature (Yang and Yanful, 2002; Lee et al., 2003; Smits et al., 2011) and matric suction (Lee et al. 2003; Smits et al., 2011). However, the evaporation test of larger soil sample in laboratory is often limited by the range and accuracy of the balance used. The lysimeter is usually used for the in-situ measurement, and the atmospheric conditions cannot be controlled. The tunnel system presents a good control of air conditions (wind velocity, temperature and relative humidity) while it is relatively expensive. Therefore, the large-scale environmental chamber seems to be a good tool for investigating the soil water evaporation in the laboratory. The chamber can measure the potential evaporation as the evaporation pan if water is poured in it (e.g., Ta, 2009). Compared to the wind tunnel system, the environmental chamber is less expensive and easier to operate; meanwhile, it can provide rich data involving both air and soil parameters. Moreover, it has the same function as the combination of the wind tunnel and lysimeter. However, most existing environmental chambers only have a good performance in controlling air conditions, the soil being hardly taken into account (e.g., Kohsiek, 1981; van de Griend and Owe, 1994; Aluwilhare and Watanabe, 2003). For the chamber developed by Ta (2009), Ta et al. (2010) and Cui et al. (2013), the evolution of volumetric water content was not well described in the near surface zone due to the limited number of sensors installed in this zone. In addition, the relationship between the actual evaporation and the soil suction or water content on the soil surface has been rarely studied.
1.3 The process of soil water evaporation and its influencing
factors
1.3.1 The requirements for the initiation of evaporation
The initiation of evaporation process needs to meet three requirements (Hillel, 2004; Lal and Shukla, 2004; Qiu and Ben-Asher, 2010):
(1) A continuous supply of evaporative energy;
(2) A vapor pressure gradient existing between the evaporating surface and atmosphere, and the vapor being transported away by diffusion and/or convection; (3) A continual supply of water from the interior of soil to the evaporating surface. In general, water is transported to evaporating surface through the soil body; the evaporation process is governed by soil water content, suction gradient and conductive properties (Hillel, 2004). The liquid water at evaporating surface is turned into vapor when there is enough energy supplied at this surface. The energy supplied is used to meet the requirement for water vaporization (i.e., latent heat, 2477 kJ/kg at 10 °C). This energy can be supplied by the surroundings or the soil body itself (Lal and Shukla, 2004). For the surroundings, the energy mainly comes from radiation or advection (e.g., solar energy). For the experiment carried out in the laboratory, this energy can be supplied by lamps (Yamanaka et al., 1997), hot air (Ta et al., 2010; Cui et al., 2013), lighting system (Lee et al., 2003) and halogen lamp (Wang, 2006). Note that, use of the energy supplied by soil body results in a temperature decrease in it (see Fig. 1.39). The vapor pressure gradient drives the vapor to the atmosphere, and the wind passing through the evaporating surface enhances this process. The mass transfer model can be used to describe this progress clearly and will be described in Section 1.4.
1.3.2 The typical process of evaporation
The evaporation process is initiated if the aforementioned conditions are fulfilled. Typically, three distinct stages can be observed (Hillel, 2004; Lal and Shukla, 2004; Wilson et al., 1994; Yanful and Choo, 1997; Qiu and Ben-Asher, 2010), as follows:
(1) The constant-rate stage
Shahraeeni et al. (2012) termed stage 1 as “the period where water is supplied to the
evaporation plane at the surface via continuous liquid pathways driven by capillary gradients acting against gravitational pull and viscous losses”. Actually, this stage
occurs at the initiation of evaporation when the soil is wet (saturated or nearly saturated state) and there are enough water supplied to the evaporating surface. Therefore, the evaporation rate in this stage is similar to that from free water. Accordingly, the evaporation rate corresponds to the potential evaporation rate. During this stage, the evaporation rate is controlled by the atmospheric conditions (e.g., solar radiation, wind speed, air temperature, relative humidity, etc.). Generally, the evaporation rate during this stage will sustain a constant value when the atmospheric conditions are steady. However, some experimental results show that the evaporation rate can remain constant for a long time under low atmospheric demand (typically ＜5 mm/day, low air speed, thick boundary layer) while it exhibits continuous decrease under high atmospheric demand (high air velocity) even in the absence of internal capillary flow limitations (Shokri et al., 2008; Shahraeeni et al., 2012). On the other hand, the duration of constant-rate stage can last a few hours or days in dry climate, and it is also affected by the evaporation rate at the initiation of this stage (Gardner, 1959; Gardner and Hillel, 1962; Yanful and Choo, 1997; Hillel, 2004).
(2) The falling-rate stage
This stage occurs when the water transfer cannot meet the requirement for sustaining the maximum evaporation rate. The evaporation decreases gradually during this stage. The quantity of water that can be conducted to the evaporating surface determines the evaporation rate. Therefore, the soil hydraulic properties play a key role in this stage. (3) The slow-rate stage
As indicated by Hillel (2004), this stage occurs when the soil surface is sufficiently dry and the liquid water transfer through it effectively ceases. The soil evaporation occurs in the zone below the dry soil layer, and the water vapor is diffused into atmosphere through this dry zone. In this case, the evaporation rate is controlled by the vapor diffusivity of the dry soil layer (Wilson, 1990). Note that this stage will persist for long time with a low rate.
The results of typical three stages are presented in Fig. 1.24. Qiu and Ben-Asher (2010) conducted evaporation experiments on sand and clay in a well insulated and air controlled chamber (Qiu et al., 2006), the results clearly exhibit the three-stage evaporation process. Figure 1.24 shows the results on the clay. At the constant-rate stage, the evaporation rate is around 0.45 mm/h. Then, it declines gradually to a value as low as 0.03 mm/day from t = 130 h to t = 412 h. This stage corresponds to the falling-rate stage. After this, the evaporation rate decreases slowly with a very low value until the end of experiment. This is the last stage of evaporation. Note that similar three-stage evaporation was observed by other authors (e.g., Wilson, 1990; Wilson et al., 1994; Yanful and Choo, 1997). On the other hand, a soil evaporation transfer coefficient, the ratio of the difference between drying soil surface temperature and air temperature to the difference between the reference dry soil temperature and air temperature, was introduced to describe the three stages (Qiu and Ben-Asher, 2010). This parameter is constant and low during the constant-rate stage. However, the cumulative evaporation increases sharply. Furthermore, this parameter and the cumulative evaporation increase with a curvilinear relationship during the falling-rate evaporation stage. At the slow-rate stage, this parameter approaches to 1 while the cumulative evaporation increases little. The value of this parameter is larger than 0.9 at the end of the second stage. Therefore, at the boundary between the last two stages, this parameter has a high value.
Fig. 1.24. The three stages of clay evaporation rate (closed circles represent the experimental data and open circles represent the simulation results) (Qiu and Ben-Asher, 2010)
1.3.3 The factors influencing soil water evaporation
It is well recognized that soil water evaporation is function of both soil physical parameters and atmospheric conditions, such as soil water content, soil microstructure, air relative humidity, air temperature, air turbulence, and especially the soil-atmosphere interface property (Philip, 1957; van Bavel and Hillel, 1976; Fukuda, 1955; Farrell et al., 1966; Scotter and Raats, 1969; Ishihara et al., 1992; van de Griend and Owe, 1994). In this section, the parameters that affect soil water evaporation and the responses of soil to evaporation are depicted.
1.3.3.1 The wind speed
Wind speed is one of the atmospheric conditions. The wind can blow away water vapor and accelerates the evaporation process. Kondo et al. (1992) used a model to investigate the relationship between the latent heat flux and wind speed (see Fig. 1.25). The latent heat flux decreases along with the decline of wind speed (16 m/s, 8 m/s and 4 m/s) at the constant rate stage, and the difference between them are very large. However, in the latter half period (after 5 days), the latent heat flux increases follow the decrease of wind speed, the difference between them being small. Therefore, Kondo et al. (1992) concluded that the evaporation rate in the initiation period is more sensitive to wind speed than in the latter half period. Meanwhile, Kondo et al. (1992) attributed this result to changes in soil resistance. The soil resistance to water transportation is small at the constant-rate stage when soil is wet, and the evaporation rate is almost determined by the aerodynamic resistance thus it is sensitive to wind speed. However, when the soil become dry, the soil resistance becomes higher as compared with the aerodynamic resistance and the evaporation rate is governed by the soil hydraulic properties. Therefore, the evaporation rate is less sensitive to wind speed. Note that the evaporation rate can be obtained by dividing the latent heat flux by latent heat of vaporization. Moreover, Yamanaka et al. (1997) performed several evaporation experiments in a wind tunnel under various atmospheric conditions. The relationship between the observed latent heat flux and the estimated evaporating surface depth at different wind speeds is presented in Fig. 1.26. Similar to the results observed by Kondo et al. (1992), the results of Yamanaka et al. (1997) also show that the evaporation at high wind speed is greater than that at low wind speed when the soil is wet but the reverse relation can be observed when the soil is dry (see Fig. 1.26). The reason of this phenomenon may be related to the energy partition between the
latent heat and sensible heat fluxes (Yamanaka et al., 1997). Note that the evaporating surface depth reflects the evaporation process.
Fig. 1.25. Wind speed effect on the daily averaged latent heat flux (Kondo et al., 1992)
0 2 4 6 8 10 12 0 100 200 300 400 500 3 m/s 1 m/s Laten t heat flux (W m -2 )
Estimated evaporating surface depth (mm)
Fig. 1.26. Relationship between the observed latent heat flux and the estimated evaporating surface depth at two different wind speeds (Yamanaka et al., 1997)
In addition, Wang (2006) conducted saturated and unsaturated soil evaporation experiments in a wind tunnel for investigating the effect of atmospheric conditions on the evaporation rate. Figure 1.27 exhibits the relationship between the potential evaporation rate and wind speed. At different net radiations, the potential evaporation increases linearly with the wind speed varying from 0 to 10 m/s. Wang (2006) considered that the wind can quickly transport the water vapor to the atmosphere, thus increasing the evaporation rate.
0 2 4 6 8 10 0 5 10 15 20 25 30 0 W/m2 200 W/m2 400 W/m2 600 W/m2 Potenti al evapora tion ra te (m m /day) Wind speed (m/s)
Fig. 1.27. Relationship between potential evaporation rate and wind speed at various net radiations (Wang, 2006)
1.3.3.2 The net radiation
0 200 400 600 0 10 20 30 0 m/s 2 m/s 6 m/s 10 m/s Po te ntia l eva pora tion ra te (m m /da y) Net radiation (W/m2)
Fig. 1.28. Relationship between potential evaporation rate and net radiation under various wind speed conditions (Wang, 2006)
In general, enhancing net radiation can supply more energy to soil and thus increases evaporation rate. However, the effect of net radiation on evaporation is affected by wind speed at the same time. The relationship between potential evaporation rate and net radiation at various wind speeds is shown in Fig. 1.28. At high wind speeds (larger than 2 m/s in this experiment), the potential evaporation rate increases gradually with the enhancement of net radiation at a given wind speed. However, under lower wind speeds, the increase of evaporation rate is not obvious. Similar results can be observed
from Fig. 1.27. Wang (2006) explained this phenomenon by the fact that low wind speeds cannot transport vapor from saturated soil to the air immediately as opposed to high wind speeds.
1.3.3.3 The relative humidity and air temperature
The air relative humidity can affect the vapor pressure gradient between evaporating surface and atmosphere, thus, affecting the evaporation process. Kayyal (1995) carried soil a column evaporation test in an oven for investigating the effect of relative humidity on the evaporation process. The temperature in the oven was controlled at 60 °C, and the relative humidity was kept at 3 %, 30 % and 43 %, respectively. The relationship between moisture loss (evaporation rate) and relative humidity is presented in Fig. 1.29. The effect of relative humidity on evaporation process is mainly identified at the first stage, i.e., constant-rate stage. As observed in Fig. 1.29, the high relative humidity corresponds to the low initial constant evaporation rate. For
example, the initiation evaporation rate is around 0.025 ml/cm2/min at a relative
humidity of 3 %; the rate decreases to 0.005 and 0.0025 ml/cm2/min when the relative
humidity values are 30 % and 43 %, respectively. On the other hand, the lower the relative humidity, the shorter the duration of constant-rate stage.
Fig. 1.29. Moisture loss rates at different relative humidity values(Kayyal, 1995) As far as the effect of air temperature is concerned, Kayyal (1995) pointed out that the vapor pressure gradient between the evaporating surface and the air increases with the increase in temperature difference between them, and in turn raises the rate of moisture leaving from the surface thus the evaporation rate in the constant-rate stage.
1.3.3.4 Soil texture
Soil texture has a great influence on the evaporation process. Noy-Meir (1973) reported that water loss during evaporation from fine-grained soils is larger than from coarse-grained soils due to the fact that the former can sustain more water than the latter. But the evaporation duration of coarse soils are shorter than the fine ones (Jalota and Prihar, 1986). Hillel and van Bavel (1976) investigated the impact of soil texture (sand, loam and clay) on the cumulative evaporation. They reported that under the same condition the fine-textured (clayey) soils make the constant-rate stage longer with a large cumulative evaporation, while the coarse-textured (sandy) soils has a short constant-rate stage with limited cumulative evaporation (see Fig. 1.30).
Fig. 1.30. Cumulative evaporation of various soils under same conditions (Hillel and van Bavel, 1976)
On the other hand, for investigating the effect of soil texture on the evaporation process, Wilson (1990) conducted various soils evaporation experiments in the laboratory. Four different types of soils (i.e., Regina clay, Botkin silt, Silica sand and Potash slimes) were used in these experiments. The soil samples were dried from slurry-saturated state to completely dry state in each shallow metallic pan (325 mm×230 mm×50 mm) at room temperature (21 °C to 23 °C) with a relatively constant relative humidity (8 % to 16 %). Furthermore, the evaporation rate from the same pan with water only was considered as the potential evaporation rate. The relationship between the ratio of evaporation rate and elapsed time is shown in Fig. 1.31. The experimental results evidence a significant effect of soil texture on evaporation. The actual evaporation rates for different textures (clay, silt and sand) are equal to the potential evaporation rate at the initiation of evaporation (constant-rate
stage); the rates of silt and sand quickly fall to zero after six days; however, the rate of clay gradually declines for a much longer time. The distinguished performance of the evaporation of Potash slimes is attributed to the use of brine for making it more slurry (Wilson, 1990). Note that the other soils are prepared with distilled water.
1.3.3.5 The hydraulic conductivity of soil
Wilson et al. (1994) investigated the effect of saturated hydraulic conductivity on the evaporation process based on a soil-atmosphere model. The relationship between the evaporation process and drying time is shown in Fig. 1.32. The saturated hydraulic conductivity has significant influence during the constant-rate and the falling-rate stages. For example, the duration of the constant-rate stage lasts 1, 4 and 6 days when
the corresponding values of saturated hydraulic conductivity are 4×10-6 m/s, 3×10-5
m/s and 8×10-5 m/s, respectively. On the contrary, the slow-rate stage is not affected by the saturated hydraulic conductivity because vapor diffusion controls the evaporation process in this stage (Wilson et al., 1994).
0 5 10 15 20 25 0.0 0.5 1.0 1.5 E va por ati on ra te ra tio Time (day) Regina clay Botkin silt Silica sand Potash slimes
Fig. 1.31. Ratio of actual evaporation rate and potential evaporation rate versus elapsed time (Wilson, 1990)
0 4 8 12 16 20 0 2 4 6 8 10 Ev ap orati on ra te (m m/ day ) Time (day) Potential evaporation Ksat = 4×10-6 m/s Ksat = 3×10-5 m/s Ksat = 8×10-5 m/s
Fig. 1.32. Evolutions of computed evaporation rates with different saturated hydraulic conductivities (Wilson et al. 1994)
1.3.3.6 The water table and drainage process
Generally, the drainage process resulting from water table decline decreases the soil water content and soil hydraulic conductivity. Therefore, the evaporation rate decreases (Yang and Yanful, 2002). For investigating the interaction of evaporation and drainage under different water table conditions, Yang and Yanful (2002) conducted a series of experiments with different cover soils (i.e., clayey till, coarse sand, fine sand and silt). Various soil columns were firstly saturated and then subjected to evaporation and drainage with different water tables. The evolutions of evaporation rate with different soils under different water table conditions are shown in Fig. 1.33 and Fig. 1.34. Note that the water tables at 0.25 m above soil bottom, at soil bottom and at 1 m below soil bottom are respectively termed as “0.25 m”, “0 m” and “-1 m” in these figures. The experimental results show that the drainage process significantly affects the evaporation progress. The evaporation rate of sands decreases along with the lowering of water table. The extent of this effect on silt is lesser than on sands. However, the clayey till is rarely affected by the water table change and water drainage process. Yang and Yanful (2002) considered that the water table and the drainage process affect evaporation through the induced suction changes. Furthermore, a deeper water table can extract more water from soil via drainage process and decrease the hydraulic conductivity, resulting in a low evaporation rate.
0 5 10 15 20 25 30 35 0 5 10 15 20 Eva pora tion ra te (m m /d)
Elapsed time (day)
Coarse sand (0.25 m) Coarse sand (0 m) Coarse sand (-1 m) Fine sand (0.25 m) Fine sand (0 m) Fine sand (-1 m)
Fig. 1.33. Evolutions of evaporation rate under different water table conditions (coarse sand and fine sand) (Yang and Yanful, 2002)
0 5 10 15 20 25 30 35 0 5 10 15 20 25 Evap orat ion rat e (m m /day) Time (day) Silt (0.25 m) Silt (0 m) Silt (-1 m) Clayey till (0.25 m) Clayey till (0 m) Clayey till (-1 m)
Fig. 1.34. Evolutions of evaporation rate under different water table conditions (silt and clayey till) (Yang and Yanful, 2002)
1.3.3.7 Effect of cracks
Clayey soil tends to swell upon wetting while it tends to shrink upon drying. During soil water evaporation, the emergency of desiccation cracks let the evaporation to be a multi-dimensional process. The water is evaporated not only from soil surface but also from cracks. On one hand, the cracks form a new way for the transportation of vapor from cracks wall to the atmosphere (Ritchie and Adams, 1974). On the other hand, the exposed vertical side of cracks can be considered as the secondary evaporating surface and hence increase the evaporation surface by three or four times (Adams and Hanks, 1964; Hillel, 2004). Obviously, the evaporation rate from cracked soils can be
also larger than the non-cracked one. The increased evaporating surface in the cracks detracts water from deeper zone, and hence affects the distribution of water content in deeper levels. A typical water distribution around a crack is shown in Fig. 1.35. Similar soil water content distribution was observed in the laboratory by Selim and Kirkham (1970).
Fig. 1.35. Water content distribution in a desiccation crack (Ritchie and Adams, 1974; cited by Hillel, 2004)
Adams and Hanks (1964) employed soil atmometers to study evaporation through natural desiccation cracks in Blackland soil (see Fig. 1.36). Note that the soil atmometers are assembly of small “moisture equivalent” boxes filled with moist soil (Adams and Hanks, 1964). The evaporation rate can be reflected by the moisture change of soil sample inside them. Adams and Hanks (1964) reported that the evaporation rate measured from soil atmometer in the deeper zone of crack was less than that in the zone close to the soil surface in the first 19 hours. Meanwhile, Adams and Hanks (1964) conducted the test of evaporation from an artificial crack for investigating the wind effect. The evaporation rate was also measured by means of soil atmometers. The experimental results show that evaporation increases at all depths as the surface wind speed increases. Furthermore, Selim and Kirkham (1970) performed soil evaporation tests with different crack widths under different drying conditions. The results show that cracks have significant influence on evaporation for fine textured soils under both wind drying and radiation drying conditions, and a crack of 0.64 cm wide increases evaporation rate by 12 %-16 % as compared to the same soil without cracks. A larger crack of 1.91-cm width can increase it by 30 %.
Fig. 1.36. Photograph of typical crack with the suspended soil anemometer (left) and the sketch of soil anemometer in the crack (right) (Adams and Hanks, 1964)
Fig. 1.37. Photograph of weighing lysimeter with a natural soil crack (Ritchie and Adams, 1974) For investigating the effect of cracks on evaporation under field conditions, Ritchie and Adams (1974) conducted evaporation experiments on bare soil with a natural crack (60-cm depth, 183-cm length) in a weighing lysimeter (see Fig. 1.37). The evaporation rate of bare soil with crack was measured firstly and an average value of 0.74 mm/day was observed. Then, the evaporation rate only from the exposed crack was monitored, and the corresponding average value recorded was 0.6 mm/day. Furthermore, the values of relative evaporation (i.e., ratio of actual evaporation rate to potential evaporation rate) from both the soil surface and the crack and only from the crack are 0.15 and 0.16, respectively. The small difference between the two cases and the nearly identical relative evaporation value demonstrate that most evaporation takes place through the crack (Ritchie and Adams, 1974; Burt et al., 2005). Similar result was obtained by Ritchie and Adams (1974) when performing an artificial crack evaporation experiment.
1.3.3.8 Soil water content
The water content of soil is an important factor in the evaporation process since it determines the quantity of water supplied to the evaporating surface. The soil evaporation experiments conducted in the wind tunnel with different initial surface water contents (Wang, 2006) showed that the actual evaporation rate decreases with the decline of initial surface water content for the same wind speed and net radiation. Wang (2006) considered that a low water content corresponds to a high soil resistance and makes the transport of water to soil surface for evaporating more difficult.
Regarding the evolution of water content during evaporation, it usually declines during the drying process (Wilson, 1990; Wilson et al., 1997; Yanful and Choo, 1997; Wythers et al. 1999), whereas it has no change during steady evaporation process. On the other hand, the results from large scale clay evaporation experiments conducted by Ta (2009) show that the loss of water is mainly limited to the surface zone (Ta, 2009; Ta et al., 2010; Cui et al., 2013). The evolution of water content during a 30-day evaporation is presented in Fig. 1.38. It can be observed that only the water content at 50-mm depth decreases from 65 % to 15 % during evaporation, while the value below 250 mm depth remains close to 50 %. Note that similar result was obtained from a clay evaporation experiment by Yanful and Choo (1997) and clayey till evaporation experiment with a water table (Yanful et al., 2003).
0 5 10 15 20 25 30 10 20 30 40 50 60 70 Volumetric w ater c ontent (%)
Elapsed time (day) 50 mm
250 mm 450 mm 650 mm 850 mm
Fig. 1.38. Evolution of volumetric water content during evaporation (Ta, 2009; Ta et al., 2010; Cui et al., 2013)
Soil temperature can change by either taking energy from the surroundings or losing energy by water evaporation. Yanful and Choo (1997) investigated the water evaporation of various soils in an environmental chamber under controlled conditions. The evolutions of soil temperature profiles for a fine sand during different evaporation stages are exhibited in Fig. 1.39. The soil temperature decreases along with the evaporation process at the initiation stage and the coldest point is at the soil surface; then it starts to increase after two days. Furthermore, the soil temperatures are nearly
constant over depths and higher than the initial temperature from 10th day to the end
of experiment. This phenomenon of soil temperature decline followed by an increase was also observed by Wilson (1990) and Wilson et al. (1997) in a sand column drying test. Wilson (1990) and Wilson et al. (1997) explained the decrease of soil temperature at the soil surface by the consumption of energy for the latent heat of vaporization. On the other hand, the evolution of soil temperature also reflects the changes in energy distribution during evaporation.
20 22 24 26 28 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 D ept h (m ) Temperature (°C) 0 1 day 2 days 3 days 10 days 41 days
Fig. 1.39. Soil temperature profile during evaporation (Yanful and Choo, 1997)
1.3.3.10 Soil suction
The soil water loss results in an increase in soil suction. A typical evolution of soil suction is shown in Fig. 1.40, obtained by Ta (2009), Ta et al. (2010) and Cui et al. (2013) using an environmental chamber. The suction at 50-mm depth increases quickly because of the significant water loss in this position (Fig. 1.38), reaching a value higher than 4000 kPa at the end of test. The suction decreases along with the
